1. Field of the Invention
This invention relates to semiconductor manufacturing, and more particularly relates to the monitoring of semiconductor process operations which use a plasma within a chamber by observation of the light produced by the plasma.
2. Description of the Related Art
Optical Emission Spectroscopy (OES) is a process by which light emitted by a process, such as a plasma within a semiconductor reaction chamber, is analyzed to see which wavelengths are present in the light. Inferences about the process may then be drawn as a result of the intensity of the various spectral lines present in the light. For example, the presence of certain species within the chamber may be ascertained. OES may be used with any chamber which provides a window through which light present within the chamber may be observed. This allows use of OES even with processes having very corrosive gases present, or processes accomplished at very low or very high pressures. This non-intrusive nature is a significant advantage of OES.
Plasma light is frequently collected via a UV transparent optical fiber and brought into a spectrometer, where the light diffracts off a diffraction grating and is dispersed into its components. The dispersed light falls onto a linear photodetector array which measures the light intensity. The result of this is a measurement of light intensity as a function of wavelength (each position on the linear array corresponding to a different wavelength) which is sampled simultaneously. Other detectors used to analyze the spectral content of the emitted light may use a single optical detector (e.g., a photomultiplier tube) which is scanned over a range of frequencies (i.e., a "scanning monochrometer"). Other methods are also possible, such as a single detector used in combination with an optical bandpass filter.
OES has been used for end point detection during various etch operations. For example, during polysilicon etching to define gate electrodes, an underlying insulating layer (e.g., an oxide layer) is uncovered between gate electrodes when the overlying polysilicon is removed. OES may be used to determine when the polysilicon has been removed by looking for the first appearance of a spectral signal from the oxide layer as it is uncovered by the etch. In other cases when an entire layer is to be removed, such as removing remaining portions of a nitride layer or photoresist film, OES may be used to detect the absence of a spectral signal from the film layer being removed.
A ratio of wavelengths has previously been used with OES to monitor certain semiconductor processes, apparently based upon a chemical understanding of what wavelengths should be present for a given reaction. For example, a spectral signal that decreases as a reaction progresses (such as a spectral signal from a layer being etched through) may be ratioed against one that increases as the reaction progresses (such as a spectral signal from a layer that first appears when uncovered by the removal of the overlying layer).
Etch stop is a condition in a plasma chamber where etch rates are drastically reduced due to the deposition of a polymeric substance on the wafer. This substance "plugs" the contacts and vias and inhibits them from being fully etched. During etch stop conditions, the rate of polymer deposition exceeds the rate of polymer etch, so that, even for very long etch times, the vias are not completely etched. Current methods of determining whether etch stop conditions are present within the tool chamber may include running periodic test wafers, which are analyzed off-line using a scanning electron microscope (SEM) to determine the etching parameters resultant from the condition of the plasma chamber. Turnaround time on SEM-analyzed wafers is usually long enough to allow a significant number of product wafers to pass through a contaminated chamber, which results in lower product yields or lower throughput.
Etch stop is one of the main causes of preventative maintenance cleanings (PM's) on plasma chambers. But if done more often than necessary, valuable time is consumed in tool downtime and consequent tool unavailability. PM's are frequently performed after a set number of wafers, whether needed or not, due to the difficulty and time delay of ascertaining whether etch stop conditions are present within the tool chamber.
Lucent Technologies (Guinn, et al.) previously introduced a method of detecting etch stop by measuring the ratio of two wavelengths of importance to the chemistry inside the chamber on each of two kinds of test wafers which were run off-line, and then ratioing the two ratios to determine whether etch stop conditions were generally present within a tool chamber. The ratio of C.sub.2 (515 nm) to SiF (440 nm) was computed for a blanket photoresist wafer, as well as for a patterned silicon wafer and a patterned silicon oxide wafer measured immediately preceding and under the same conditions as the blanket photoresist wafer. The range of etch rates was correlated to the photoresist removal rate which was correlated to the C.sub.2 intensity. The ratio against the SiF intensity was used to correct for brightness of the plasma light. These two ratios were then ratioed together to compensate for drift of the tool over time, and the resulting "ratio-of-ratios" used to predict the etch rate of the photoresist: if too high, it indicates a resulting loss of critical dimension (CD) tolerance; and if too low, it indicates an excessive deposition of photoresist and other polymeric material that is unstrippable, namely etch stop conditions within the chamber. Such measurements were not performed on actual production wafers but required use of special test wafers run at periodic intervals.
One of the difficulties of using OES in a semiconductor process is the large number of spectral lines present within many typical plasmas, which may contain a large number of different reaction components. Each molecule within a plasma emits its own spectral signature, which may overlap the spectral signature of other molecules present within the plasma. Each species has its own fingerprint of wavelengths. Identifying the individual species is difficult, however, because the fingerprints overlap each other, and may share particular wavelengths. Attempting to analyze such data without a methodical analysis is time consuming and likely to result in less than optimal process monitoring. Moreover, utilizing expected spectral wavelengths based solely upon the chemistry of the reaction may result in false indications of process conditions such as etch stop. This may result when a spectral line is cluttered by neighboring lines from other species.
What is needed is an on-line method for monitoring a process using OES which minimizes or doesn't require use of off-line test wafers periodically processed between production runs. Moreover, what is additionally needed is a faster, more systematic selection of process-important data from a broad array of collected data, and a method which reduces the risk of selecting unreliable fault detection and monitoring signals. When applied to etch stop detection, such a capability would allow less frequent PM's and a consequently longer chamber life, because chamber cleans would be performed only when needed. Moreover, product yields could increase because fewer wafers would be processed when etch stop conditions are present and before such conditions are discovered.